Facilities that generate power by nuclear fission often use water for containment of the reaction rate and provide a certain amount of radiation shielding of the fuel in the reactor core (in pressured water and boiling water-type reactors), and to store spent fuel cells. A typical pressure water reactor (PWR) can have on the order of 280 fuel cells in its reactor core. A boiling water reactor (BWR) can have 500 fuel cells in use and in storage. A spent fuel pool can have as many as 5,000 units at any given time. Strict governmental regulation requires that the specific location of each fuel cell in a facility be known at all times. Failure to comply, even in the case of a minor variation, can result in substantial fines.
The operation and maintenance of nuclear facilities requires that the fuel cells contained in the reactor be periodically inspected and replaced. In both PWR and BWR facilities, the reactors are typically refueled every twelve to eighteen months. The refueling process removes spent fuel to the spent fuel pool, relocates existing fuel cells within the reactor core, and inserts new fuel cells. This process involves much more than simply replacing spent cells with new cells, since a significant "shuffling" of the fuel cells already within the reactor must occur in order to balance the radiation level across the reactor core to maintain uniformity. In order to comply with the strict governmental regulations, as well as to enable identification of fuel cells to determine their appropriate position within the core, means must be provided for in situ identification of each individual fuel cell with minimum exposure of personnel to dangerous radiation levels. This requires that the fuel cells be identified while still submerged within the water in the reactor containment or spent fuel pools. This presents a significant problem, however, since the depth of the water, at least in the containment pool, is on the order of 60 feet.
The industry standard is for the fuel cells to be identified with an alphanumeric serial number which is stamped, milled or engraved on the top surface of the fuel cell, or on a bail extending partially across the top of the unit. Typical depths for new characters can be 0.5 to 1.0 mm. Thus, in order to identify a particular fuel cell, it is essential that the serial number be both visible and intelligible. On new fuel cells, the contrast between the background surface and the characters in the serial number is good, and the serial number can easily be read using a common video camera, such as a camcorder. However, this contrast degrades over time due to the enhanced corrosion and/or oxidation of the surface that is caused by the radiation, and sediment buildup on the surfaces of the fuel cell, with the characters becoming as shallow as 0.07 mm.
A number of identification systems have been developed which are intended to allow the identification of fuel cells. For example, U.S. Pat. No. 4,960,984 ('984 patent) which issued to Goldenfield for an invention entitled "Method and Apparatus for Reading Lased Bar Codes on Shiny-Finished Fuel Rod Cladding Tubes," teaches the identification of nuclear fuel rod tubes that are encoded with a bar code by scanning the bar code with a bar code reader. This method requires marking a bar code on the fuel cells in addition to the industry standard alphanumeric code, which may be possible for newly manufactured cells, but could prove very difficult for used fuel cells (spent or in use). Further, the system and method of the '984 patent does not allow identification of a fuel rod tube while the tube is submerged within the reactor pool. Instead, a sophisticated machine is described which receives the fuel rod tube, and directs a laser beam towards the bar code etched in the tube. This laser beam is reflected back to the beam source for decoding to yield the identification information for the fuel cell. While this system may be capable of accurately identifying the fuel cell tube using a pre-existing engraved bar code, it is incapable of obtaining such identification while the fuel cell is submerged within the reactor pool.
Another fuel cell identification system is briefly disclosed in U.S. Pat. No. 5,490,185 ('185 patent), issued to Dent, et al., for an invention entitled "System for Automatic Refueling of a Nuclear Reactor," which includes the use of an optical scanner to identify the bar code or alphanumeric code. While this system includes video capability for identification of the fuel cell while still submerged within the reactor pool, the video feature is described simply as a remotely operated video camera which is attached to the fuel handling equipment. Without special considerations for contrast enhancement, corrosion on the outside surface of the nuclear fuel cells and the buildup of corrosive materials on the surface of the nuclear fuel cell will inhibit the video determination of any identifying markings.
Yet another identification system is disclosed in U.S. Pat. No. 5,089,213 ('213 patent), issued to Omote, et al., entitled "Nuclear Fuel Assembly Identification Code Reader." The '213 patent discloses the use of a combination of a camera and an ultrasonic wave sensor for what is described as a more reliable process for identification of the fuel cell. The device disclosed in the '213 patent combines the data from the camera and the acoustic device to provide the identification, requiring a relatively complex processing program. The two different reading techniques complement each other in an effort to overcome the inadequacies of each system individually. Thus, a significant loss in contrast in the characters would require the system to rely almost exclusively on the acoustic component of the system. The combination of the processing requirements and the multiple independent detection components would make this system relatively complex and expensive
In light of the above-stated inadequacies of the prior art, it would be desirable to provide a system and method for in situ underwater inspection of the nuclear fuel cells in reactor pools and spent fuel pools that is capable of reading existing industry-standard identification characters in which the system is capable of reading the characters in spite of the inevitable degradation of the characters caused by corrosion. It is to such a system that the system and method disclosed herein is directed.